The invention is based on a fuel injection system for internal combustion engines.
In a known fuel injection system of this type (German Offenlegungsschrift 30 11 097), the control chamber can be relieved in a throttled fashion via a control line the cross section of which is controlled by the electrically actuated valve, so that during the feed of the injection pump the blocking piston yields accordingly. The blocking piston, functioning like a reservoir, receives some pumped fuel. After the pump feed has ended, the blocking piston, driven by the restoring force, pumps this quantity of fuel to the injection nozzle, where it is injected; this assures that this quantity will also be supplied to the engine. The result is a prolongation of the injection duration, bringing about quieter engine idling.
The quantity of fuel received by the blocking piston during its deflection is equivalent to the quantity that flows, throttled, via the control valve; hence at relatively low rpm, when such quiet idling is particularly desirable, the relatively small quantities positively displaced by the blocking piston mean that relatively small control throttle cross sections must be established and maintained. This means additional problems in the event of a temperature change, because a temperature increase not only increases the viscosity but also changes the cross sections at such valves, so that a temperature change also necessitates a control correction. A further disadvantage of this system is that the control pressure is approximately subject to the injection pump feed pressure, which puts an extraordinary burden on the control chamber and control valve, with attendant disadvantages in terms of effecting control. When a plurality of fuel injection nozzles must be supplied in succession by the injection pump, there are typically differences in the opening and closing force of the nozzle, which correspondingly affects the backup pressure established in the pressure line and hence affects the force engaging the first face of the blocking piston. That, in turn, leads to differences among the injection durations of the various nozzles and above all means a relatively narrow range of tolerance for the magnitude of the restoring force engaging the blocking piston, because the return pressure of the blocking piston, which is determined by the restoring force and the first face, should be as low as possible, in order not to unnecessarily burden the outflow from the control chamber, yet it must in each case be greater than the closing pressure of the injection nozzle, in order to assure the required return feed prior to the closure of the injection nozzle.
In another injection system of this type (U.S. Pat. No. 4,546,749), the blocking piston and the control valve serve as an injection quantity determining device, in that during the fuel pumping by the injection pump the blocking piston always yields whenever the injection is to be terminated. Because of the opening of the magnetic valve, the pump feed pressure displaces the blocking piston toward low hydraulic pressure, until an outflow opening is opened by the blocking piston control edge facing toward the pump work chamber, and the remaining feed quantity of the injection pump flows out without pressure. In this case the restoring force is determined by the pressure of a feed pump upon the second face. Once again, there is the disadvantage that the control chamber and hence the magnetic valve are operating under high-pressure conditions, namely the pressure that likewise prevails in the injection pump work chamber. A further disadvantage is that the blocking piston stroke is relatively short and thus the working quantity flowing through the magnetic valve is relatively low; both these factors have a deleterious effect on control, and in particular on the accuracy of control.